Electric utility companies and power consuming industries have in the past employed a variety of approaches to metering electrical energy. Typically, a metering system monitors power lines through isolation and scaling components to derive polyphase input representations of voltage and current. These basic inputs are then selectively treated to determine the particular type of electrical energy being metered. Because electrical uses can vary significantly, electric utility companies have requirements for meters configured to analyze several different nominal primary voltages, the most common of which are 96, 120, 208, 240, 277 and 480 volts root mean squared (RMS).
Electric utility meters employing electronic components instead of electromechanical components have become more widely used in the industry. The use of electronic components including microprocessor components have made electricity metering faster and more accurate. Of course, the meters typically receive and monitor alternating current (AC) power from the power distribution system. Usually, direct current (DC) power is required to operate the meter's electronic components. Therefore, electronic meters use power supply devices to generate DC power from the already-available and constantly-present AC line voltage. As discussed in U.S. Pat. No. 5,457,621, which is incorporated herein by reference, power supply devices have been created to generate the required microprocessor DC power regardless of the value of the available line voltages (e.g., 96 to 480 volts RMS).
Unlike the former electromechanical components, electronic meters use more sophisticated electronic circuitry to accurately sense and measure AC voltage and current on the power distribution system (i.e., line voltage and line current). In order to minimize cost, however, power sensing circuitry is kept as simple as possible. For example, in lieu of more complicated circuit isolation techniques (although not excluded from scope of the invention), meters often employ power line-referenced voltage sensing techniques.
One type of power line-referenced sensing technique accomplishes voltage sensing with high value resistors in a resistive-divider circuit configuration. Resistive voltage sensing typically requires that the meter's electronic circuits use a voltage that is referenced to one side of the AC power line. In addition, the measurement of electrical power by the electronic metering circuits require that line voltage and line current be sensed very accurately. For example, typical voltage sensing accuracy is usually greater than 0.1% in order to allow the overall meter accuracy to be greater than 0.2%. Line-referenced sensing techniques, like resistive division circuitry, insure compatibility with the sensitive electronic sensing circuits required to meet the meter's rigid measurement accuracy requirements.
Because the voltage being sensed is an alternating voltage signal, the sensed voltage (and therefore also the voltage reference provided to the meter's electronic circuitry) normally swings above and below a referenced ground level. In the context of an electronic circuit that receives DC power from a power supply, for example, often the less expensive electronic circuits (e.g., amplifiers) cannot receive a signal that drops below the power supply's negative power rail (e.g., for a 5 VDC power supply the positive rail may be +5 and the negative rail may be 0 VDC). Therefore, these electronic circuits often cannot use the AC power line's voltage because it swings below the ground-referenced level or negative power rail.
Attempting to use lower cost components and reduce power consumption to keep meter costs down is relevant to any meter environment, and particularly the residential meter environment. For example, although the invention is not limited to this application alone, 240 VAC residential meters often use limited current, fixed voltage capability power supplies (e.g., a linear capacitive divider power supply), which apply even greater power constraints on the power supply.
Therefore, there is a need to adjust or bias the AC voltage inputted to the meter's electronic circuitry so as to prevent it from swinging below the referenced ground level, while maintaining the meter's required level of measurement accuracy.